How did the dinosaurs become birds? Maybe the nose knows

General Studies Paper 3

Introduction

• In a famous 1859 book, On the Origin of Species, the British naturalist Charles Darwin presented the world with the theory that life-forms evolved through natural selection .The anatomist Thomas Henry Huxley suggested that the birds of today could be the descendants of the extinct dinosaurs.

Evolution of dinosaurs

• The evolution of the skull of dinosaurs to birds has been one of the main focuses in dinosaur palaeontology for a long time.
• By focusing on the nose, this study helped us improve our understanding of the cranial evolution of dinosaurs to birds.
• In 1998, two fossils discovered in China provided paleontological evidence that modern birds had evolved from theropod dinosaurs.
• The 120-million-year-old fossils, of the species Protarchaeopteryx and Caudipteryx, depicted different stages in the evolution of birds from terrestrial, two-legged dinosaurs with feathers.
• The dinosaurs had slowly developed bird-like features, such as wings, wishbones, and feathers.
• Even today, scientists are not sure whether dinosaurs were warm-blooded or cold-blooded.
• The word ‘dinosaur’ comes from the Greek words ‘deinos’ and ‘sauros’, meaning “terrible lizard”, and lizards are ectotherms.
• On the other hand, dinosaurs are also related to birds, which are warm-blooded.
• The location of non-avian dinosaurs in the phylogenetic tree – a diagram depicting how different life-forms are related to each other by evolution – is somewhere between animals that depend on environmental conditions to regulate body temperature (e.g. lizards and crocodiles) and those that can regulate it on their own (e.g. birds and humans).

Ask the nose

• The nasal cavity of warm-blooded animals houses a complex scroll-like structure made of thin bony plates called the nasal, or respiratory, turbinate.
• Nasal turbinates are found only in warm-blooded creatures. They are responsible for regulating heat and moisture exchange during respiration.
• Because of their gelatinous composition, nasal turbinates rarely survive fossilisation.
• The team obtained computed tomography (CT) scans of 51 present-day species: 21 birds, eight mammals, four Crocodylia (crocodiles and alligators), three Testudines (turtles and tortoises), and 11 Lepidosauria (snakes, lizards, iguanas, etc.). The scan data was then used for 3D reconstruction of their nasal cavities.
• The team also digitally reconstructed a three-dimensional nasal cavity of a velociraptor (a type of theropod) based on fossils.
• Comparing these 3D scans, the researchers found that, relative to the size of their heads, warm-blooded animals had much larger nasal cavities than cold-blooded animals.

World of the dinosaurs

• The reconstruction and some analysis also shed light on a lesser-known physiological function of respiratory turbinates: brain-cooling.
• The study discovered that one of the primary functions of the respiratory turbinate and the bigger nasal cavity of [warm-blooded animals] is to cool their larger brains, not for whole-body metabolism.
• The researchers also found that the velociraptor had a smaller nasal cavity than that of modern birds, and that the theropod didn’t possess a fully developed cooling system that would be required for a brain that could ‘operate’ a warm-blooded animal.
• Birds and mammals on the other hand had large nasal cavities that in turn accommodated a well-developed respiratory turbinate, and that in turn cooled their brains efficiently.
• They also found that in the velociraptor, the maxilla had a significant influence on the shape of the nasal passage.
• Based on this, they have proposed that a great reduction of the maxilla on the theropod lineage resulted in the nasal cavity becoming an important apparatus for their thermal regulation strategy.

Conclusion

• Organisms do not evolve in a vacuum but in relation to their surrounding environment. The study of evolution of dinosaurs and the environment of the earth they walked on will be crucial in understanding the lineages that lead to each phylum or class in the Animal kingdom.